36 FISH OF THE OLD RED SANDSTONE. 



a heterocercal tail, the caudal fin large, obliquely subtruncate or 

 slightly concave in the middle of the posterior margin, and con- 

 fined to the lower side of the spine. I find, from the examination 

 of probably more perfect specimens than were at the disposal of 

 Prof. Agassiz, that the tail of this genus really presents a very 

 different and peculiar structure ; so far from being truly hetero- 

 cercal*, there is almost as great a development of fin -rays above 

 as below the spi- 

 nal prolongation, 

 the caudal fin ha- 

 ving a rhomboidal 

 form, the poste- 

 rior margin point- 

 ed in the middle ; 

 the spinal prolon- 

 gation is much at- 

 tenuated_, reach- 

 ing nearly to the 

 extremity of the 

 fin ; it is not pre- 

 cisely in the mid- ^. , 1 * M/r.- 7 . X 

 die, but a little D.phycercal tail iD^plopterns). 



above it, as in the sketch. This form of tail I find in all the 

 species of Diplopterus (Ag.), and also in Gyroptychius (M'Coy), 

 and as it is in some measure intermediate in appearance be- 

 tween the " homocercal " and " heterocercal '^ types, though 

 possessing some structural peculiarities of its own, I would pro- 

 pose to designate it in the descriptions by the term " diphycer- 

 caP^ (from Btcfyvr)^, duas hahens naturas, and KepKo^, cauda). In 

 the " homocercal " or ordinary form of tail of most recent and 

 the newer fossil fishes, we usually find a few of the last vertebrse 

 anchylosed, and from the terminal mass so formed the greater 

 portion of the caudal fin is developed, as much from the upper 

 as from the lower aspect, and only the few short rays at the 

 commencement of the fin being intercalated with the spinous 

 processes of the preceding normally-formed vertebrae; in the 

 " heterocercal " type the vertebrae do not coalesce into a terminal 



* Some of the recent examples of heterocercal tails do not present the 

 strongly-marked characters of the older fossil Ganoids ; 1 allude particularly 

 to the sharks, which when the skin is removed show a fringe of fin-rays 

 above the spine, although much smaller than the great fins developed from 

 the under side, thus making an approach to our fossil " diphycercal " type ; 

 this has heen also noticed by Prof. Miiller of Berlin, who hence remarks 

 that the heterocercal and homocercal types pass into each other ; in the fossils 

 however the distinction is generally speaking a very marked and valuable 

 one, and is scarcely affected by the discovery of the peculiar structure above 

 illustrated, the notice of which will on the other hand remove some erro- 

 neous impressions. 



